1. Field of the Invention
This invention relates generally to Digital Subscriber Line (DSL) systems affected by TCM-ISDN crosstalk noise, and in particular to canceling at least some of the TCM-ISDN crosstalk noise.
2. Background of the Invention
Under certain operating conditions, Digital Subscriber Line (DSL) transmissions can be affected by crosstalk interference from other services bundled within a common cable binder. The level of crosstalk generated by other services varies for different cable structures and materials. Some countries, such as Japan and Korea, use telephone cables with a paper-based “pulp” insulator rather than the plastic insulated cables (PIC) used in the United States. These pulp cables have poor insulation and thus cause a high level of crosstalk between different services over copper wires bundled in the same cable binder. Integrated Services Digital Network (ISDN) service is especially troublesome when combined with DSL service, such as Asymmetric DSL (ADSL), because portions of the transmission band for ISDN service overlap portions of the transmission band for DSL service. Like DSL service, ISDN service is deployed widely over copper wires and bundled in the same cable binders as the wires used in DSL service. Because the transmission bands for ISDN and DSL services overlap, ISDN service can cause crosstalk in and interfere with DSL services.
A special system was developed to address this problem where the noisy pulp cables are installed, described in the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) specification G.961 Appendix III. The G.961 Appendix III system reduces crosstalk interference by switch synchronizing ISDN cards at the central office using Time Compression Multiplexing (TCM). TCM provides for ISDN signal transmission and reception during different time periods to reduce near-end crosstalk between ISDN services. ITU-T ADSL standards G.992.1 Annex C and G.992.2 Annex C describe the operation of DSL modems in the presence of TCM-ISDN interference. According to these standards, signal transmissions from DSL modems are switch synchronized to a 400-Hz TCM Timing Reference (TTR) generated at the central office. The TTR signal is the master clock signal for determining when the central office (CO) modem and the customer premises equipment (CPE) modem should transmit and receive ISDN and DSL signals.
Within a particular cable binder, the TCM-ISDN system results in a time-varying noise environment. During the first half period of the TTR signal, the CO modem is dominated by near-end crosstalk (NEXT) interference, and during the second half by far-end crosstalk (FEXT) interference. The reverse is true for the CPE modem. Because FEXT interference is much weaker than NEXT interference and is smaller relative to the received signal, the signal-to-noise ratio (SNR) in the presence of FEXT is higher than in NEXT. Channel capacity and potential data rates are thus higher during FEXT periods and lower during NEXT periods. When the loop is relatively long, in fact, NEXT periods are often unusable due to the TCM-ISDN crosstalk, which may significantly degrade the performance of the DSL transceiver. Accordingly, it is desirable to reduce the noise in a DSL line caused by the TCM-ISDN crosstalk.